Mechanical watch designers have for a long time devised numerous devices for absorbing the energy resulting from a shock, particularly a lateral shock, by the balance staff abutting against a wall of the hole in the base block through which it passes, while allowing a momentary movement of the pivot-shank before it is returned to its rest position via the action of a spring.
FIGS. 1 and 2 illustrate a device, called a “reverse double cone”, which is currently used in timepieces on the market. A device of this type is for example disclosed in FR Patent No. 1 532 798.
A support block 1, whose base comprises a hole 2 for the passage of balance staff 3 ending in a pivot-shank 3a, allows the positioning of a setting 20 in which a pierced stone 4, through which the pivot-shank 3a passes, and an endstone 5 are stopped. Setting 20 is held in a recess 6 of support block 1 by a spring 10, which in this example includes radial extensions 9 compressing endstone 5. Recess 6 comprises two shoulders 7, 7a in the shape of inverted cones which support complementary shoulders 8, 8a of setting 20, said shoulders having to be made with a very high level of precision. In the event of an axial shock, spring 10 acts alone to return balance staff 3 to its initial position. In the event of a lateral shock, i.e. when the end of the pivot-shank unbalances setting 20 outside its rest plane, spring 10 cooperates with the complementary inclined planes 7, 7a; 8, 8a to centre setting 20 again.
In this construction it is very difficult to adjust the compression force of spring 10 and the optimum angle of the complementary inclined planes 7, 7a; 8, 8a at the same time, taking account also of the friction coefficient between said inclined planes, such that re-centring errors can occur after a lateral shock. This obviously has the drawback of adversely affecting the chronometric qualities of the timepiece.
Shock absorber devices that do not include a setting and thus without any of the aforementioned problems of friction, have also been proposed in the past. CH Patent No. 237 812 for example, discloses a device comprising a pierced stone secured to a first resilient device clamped in the block by means of a slit ring and an endstone held by a second resilient device formed by a star-spring. The presence of the two springs making the two stones move separately raises re-centring problems in the event of a shock and problems of lubrication. In CH Patent No. 577 202, the two stones are also suspended between two washers comprising diverging radial resilient strips respectively abutting on the endstone and on the pierced stone, the two stones being separated by a separating washer, which also raises re-centring problems because of friction between the stones and the resilient strips.